Carbon fibers are used in various applications owing to their excellent mechanical properties and electric properties. The conventional applications of carbon fibers include sporting goods such as golf clubs and fishing rods and aircraft, and in recent years, in addition, carbon fibers are increasingly used in so-called general industrial applications as automobile members, compressed natural gas (CNG) tanks, aseismic reinforcing members of buildings and ship members. This tendency requires higher levels of mechanical properties. For example, in the aircraft application, many of the structural members are being replaced by carbon fiber reinforced plastics lighter in weight, and carbon fibers having both high compressive strength and high tensile elastic modulus are being demanded.
The industrial production of carbon fibers undergoes a stabilization step of heat-treating precursor fibers made of a polymer such as polyacrylonitrile in air with a temperature of 200 to 300° C. and a carbonization step of heat-treating the stabilized fibers obtained in the stabilization step in an inert atmosphere with a temperature of 300 to 3,000° C. In general, if the highest temperature in the carbonization step is higher, the tensile elastic modulus of the obtained carbon fibers can be made higher, but since graphite crystals grow, the compressive strength of the obtained carbon fibers decline. That is, there is trade-off relation between the tensile elastic modulus and the compressive strength of carbon fibers. To overcome the trade-off relation, several techniques for enhancing the compressive strength and the tensile elastic modulus have been proposed in addition to those for controlling the carbonization temperature.
Techniques proposed for enhancing the compressive strength of carbon fibers include, for example, a technique of implanting ions into carbon fibers, for making graphite crystals amorphous and a technique of using precursor fibers with a non-circular cross sectional form for increasing the geometrical moment of inertia (see JP 3-180514 A and JP 3-185121 A). However, the former proposal allows carbon fibers to be treated only little by little in high vacuum, and the latter proposal has a problem in view of uniformity of final products, since it is difficult to maintain the sectional form stably. Both the proposals involve difficulty in industrial application.
To enhance the tensile elastic modulus of carbon fibers, as is known, it is effective to draw fibers at the time of stabilizing-carbonizing treatment, for enhancing the orientation degree of carbon fibers. However, merely enhancing the draw ratio causes fuzz generation and fiber breakage, and it cannot be, avoided that the production stability and the grade of the obtained carbon fibers decline. Techniques for stabilizing drawing by controlling stabilizing-carbonizing treatment conditions are also proposed (see JP 2004-91961 A and JP 2004-197278 A). However, the drawing level achieved is not satisfactorily high, while the effect of enhancing the tensile elastic modulus by drawing is slight.
A technique for enhancing oxygen permeability of the precursor fibers to be used and homogenizing reaction among, the single filaments in the stabilization step, to thereby enhancing the tensile elastic modulus of the obtained carbon fibers, is proposed (see JP 2-84505 A). However, this proposal has a problem that, since as much as more than 1.5% of a comonomer is used for enhancing the oxygen permeability, the heat resistance of the precursor fibers declines, though the effect of enhancing the tensile elastic modulus can be certainly obtained. The decline of heat resistance causes more single filaments to adhere to each other in the drying heat treatment step or the steam drawing step of the fiber producing step, and in the stabilizing or carbonizing in the stabilizing-carbonizing treatment step, to lower the production stability and the tensile strength and compressive strength of the obtained carbon fibers.
It could therefore be helpful to provide carbon fibers excellent in both compressive strength and tensile elastic modulus without impairing productivity and processability, and also to provide a production process thereof. It could also be helpful to provide a process for producing polyacrylonitrile-base precursor fibers used for production of the carbon fibers.
We obtain carbon fibers by spinning a polyacrylonitrile-base polymer and subsequently using a stabilizing-carbonizing treatment so that the carbon fibers have a strand tensile modulus of 320 to 380 GPa and a conduction electron density of 3.0×1019 to 7.0×1019 spins/g as determined by electron spin resonance.
In the carbon fibers, it is preferred that the strand tensile modulus is 330 to 380 GPa and that the conduction electron density as determined by electron spin resonance is 4.0×1019 to 7.0×1019 spins/g.
In the carbon fibers, it is preferred that the crystal size of the carbon fibers is 1.8 to 2.6 nm.
In the carbon fibers, it is preferred that the specific gravity of the carbon fibers is 1.75 to 1.85.
In the carbon fibers, it is preferred that the average single filament diameter of the carbon fibers is 4.5 to 7.5 μm.
The process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers for achieving the abovementioned objects, which comprises spinning a spinning dope containing 10 to 25 wt % of a polyacrylonitrile-base polymer having an intrinsic viscosity of 2.0 to 10.0 by extruding it from a spinneret by a wet spinning or dry wet spinning method, drying and heat-treating the fibers obtained in the spinning step, and steam-drawing the fibers obtained, wherein the linear extrusion rate of the polyacrylonitrile-base polymer from the spinneret is 2 to 15 m/min.
In the process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers, it is preferred that the linear extrusion rate is 2 to 10 m/min.
In the process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers, it is preferred that the spinning method is a dry wet spinning method.
In the process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers, it is preferred that the melting point Tm in wet heat of the polyacrylonitrile-base polymer measured by a differential scanning calorimeter is 186 to 200° C.
In the process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers, it is preferred that the polyacrylonitrile-base polymer is a copolymer containing a component copolymerizable with acrylonitrile and that the amount of the copolymerizable component is 0.1 to 0.5 mol %.
In the process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers, it is preferred that the single filament fineness of the polyacrylonitrile-base precursor fibers for production of carbon fibers is 0.7 to 1.0 dtex.
The process for producing carbon fibers comprises stabilizing the polyacrylonitrile-base precursor fibers for production of carbon fibers, produced by the process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers, in air with a temperature of 200 to 300° C., while drawing them at a draw ratio of 0.80 to 1.20, preliminarily carbonizing the fibers obtained in an inert atmosphere with a temperature of 300 to 800° C., while drawing them at a draw ratio of 1.00 to 1.30, and carbonizing the fibers obtained in an inert atmosphere with a temperature of 1,000 to 2,000° C., while drawing them at a draw ratio of 0.96 to 1.05.
In the process for producing carbon fibers, it is preferred that the draw ratio in the stabilization step is 0.90 to 1.20, that the draw ratio in the preliminary carbonization step is 1.10 to 1.30, and that the draw ratio in the carbonization step is 0.97 to 1.05.
If the polyacrylonitrile-base precursor fibers for production of carbon fibers, produced by the process for producing polyacrylonitrile-base precursor fibers for production of carbon fibers are used, fibers can be drawn highly and stably in the carbon fiber production process without impairing the productivity and processability of the carbon fiber production process. As a result, carbon fibers excellent in compressive strength and tensile modulus, and excellent further in tensile strength and grade can be produced at low cost.